For sound reproduction, especially movie sound reproduction, there are different kinds of systems which differ with regard to their complexity and reproduction quality. The reference for movie sound is the cinema. Cinemas provide multi-channel surround sound, with loudspeakers installed not only in front at the screen, but additionally on the sides and rear. The side and rear loudspeakers enable an enveloping surround sound.
For the home, so-called home cinema systems usually feature five loudspeakers and a subwoofer. Three of the loudspeakers are in front and two are on the side/rear. The side/rear loudspeakers often pose a problem: People will often rather be without them to avoid not only visually distracting loudspeakers in the rear, but also the corresponding cabling.
An alternative to home cinema systems are soundbars. Many variations of soundbars exist on the market. The most sophisticated soundbars not only enhance the sound spatially, but form beams to project the sound signals to the side/rear, with the help of reflecting walls. In this case, true surround with a sound perceivable from side/rear is reproduced without surround speakers.
A soundbar projecting the sound channels to the side/rear comprises a loudspeaker array which projects at least one channel to the side/rear by means of beamforming, e.g. a delay and sum beamformer. A limitation of delay and sum beamformers is that the aperture of the array has to be at least of the size of order of magnitude of the wavelength of a sound frequency to be emitted. If the array is small compared to the wavelength, no directive beam can be formed.
For example, when a 1.2 m long soundbar emits sound at 200 Hz (wavelength 1.7 m), no beam with high directivity can be formed. Consequently, soundbars can only effectively project sound to side/rear at medium to high frequencies. Low frequencies will be reproduced from the front, since projection over walls involves very high directivity (such that only a very low level of sound is reaching the listeners directly, while most of the sound is reaching the listeners via a wall reflected beam).
The U.S. Pat. No. 8,477,951 discloses a loudspeaker array reproduction system that improves the stereo effect of middle and low frequency signals through the use of a psychoacoustic model. The input signal is split, and one part for which beamforming is not performed, is reproduced using virtualization techniques based on HRTF processing, the other part is processed using beamforming techniques. Further audio systems comprising a plurality of channels which feature a loudspeaker array are disclosed by the US Patent Application US 2005/0089182 and the U.S. Pat. No. 5,953,432.
The U.S. Pat. No. 8,189,795 discloses a processing for use of the loudspeaker array, where high and low frequency bands are reproduced in different ways. While the high-frequency part is played back using beamforming techniques, the low frequency part is further divided into correlated and uncorrelated parts, which are then played back by further non-arrayed loudspeakers with different directivity.
The U.S. Pat. No. 8,150,068 discloses an array playback system for surround sound input, that makes use of a frequency division into high and low frequency parts. The higher frequency is reproduced using the loudspeaker array for beamforming and utilizing the wall reflections. The lower frequency part of the different input channels are summed into signals which are output over one or more woofer speakers.
All above teachings have the drawback of high complexity and/or limited quality of surround reproduction. Therefore, there is a need for an improved approach.